Should I go to formal/prom???

I’m sorry this turned out longer than I thought but I really hope somebody reads it! I need help!

So my year 12 formal (like prom in America) is coming up. It’s really not my kind of thing and I was already really nervous about going anyway. My little sister is on a 3 month long trip (no phone calls or anything) and is coming back the day that the formal is supposed to be. My parents are driving to pick her up (like a 10hour drive) and I really want to be there (we’re super close) but I can’t if I go to formal. My friends and I all agreed not to bring dates this year but suddenly they’re all taking people and I’m the only one left without a date. They are making me feel awful about it and I just think the whole night is going to be a humiliating nightmare. I don’t have a dress yet and I can’t do makeup or anything so it’ll all be really expensive (plus the limos/buses that my friends want to take). I only have one real friend so I don’t want to abandon her by not going, but she actually enjoys this type of thing. I really don’t want to go but I know that everyone will think I’m weird and I probably won’t have any friends left if I don’t go. I need some advice!!! Should I go to formal or go see my sister???